Extensions to the C Language - GCC and Sunstudio

GNU C:GNU C provides several language features not found in ISO standard C.
(The -pedantic option directs GCC to print a warning message if
any of these features is used.) To test for the availability of these
features in conditional compilation, check for a predefined macro
__GNUC__, which is always defined under GCC.These extensions are available in C and Objective-C. Most of them are
also available in C++. See Extensions to the C++ Language, for extensions that apply only to C++.Some features that are in ISO C99 but not C89 or C++ are also, as
extensions, accepted by GCC in C89 mode and in C++.See more details click here

Sunstudio C 12New Language Extensions in the Sun Studio 12 C CompilerThis article gives an overview of the following C-language
extensions (part of the GNU C-implementation) introduced in the Sun
Studio 12 C compiler. Although these extensions are not part of the
latest ISO C99 standard, they are supported by the popular gcc
compilers.

The typeof keyword which allows references to an
arbitrary type

Statement expressions that make it possible to specify
declarations and statements in expressions